The invention relates generally to ignition systems, and more particularly to apparatus and methods for detecting the occurrence of spark discharges across an igniter.
Conventional ignition systems are well known and typically include an exciter having an energy storage device such as a capacitor and an circuit for charging the capacitor, one or more igniter plugs, and a switching mechanism connected between the capacitor and the igniter. In aerospace applications, the switching mechanism commonly is a spark gap, or more recently solid state switches such as SCRs. A control circuit is also provided to control when the switching mechanism is triggered so that the energy stored in the capacitor can be discharged across the igniter gap. During the time that the switching device is open, the capacitor is charged by the charging circuit.
It is often desirable to know whether the ignition system is operating properly, particularly to know if the spark rate is being maintained. For example, spark rates can be significantly affected by operating temperature excursions or variations of input voltage or frequency. Also, various failure modes within the discharge circuits can prevent proper discharge of current through the igniter. Accordingly, many ignition diagnostic systems use a current transformer to detect discharge current flowing to the igniter, typically through the high tension lead or return lead. The current transformer includes a wire coil on a high permeability core that surrounds the current lead. Discharge current through the ignition system cables induces a current in the transformer that can then be analyzed by the diagnostic system because the induced current is related to the occurrence of a spark and current discharge. The current transformer provides a way to detect not only the occurrence of a discharge, but also the corresponding energy level and duration of the discharges.
Although known current pulse detection techniques are useful, they often times are more sophisticated than what the customer needs for diagnostic purposes. In many situations, the only information needed is that a spark discharge occurred. A spark is typically characterized by a high frequency current discharge from the exciter through the high tension lead and plug. Also, current transformers and similar devices have an iron or other heavy core which usually requires a fixed mounting of the detector and adds weight to the overall ignition system and engine. In aerospace applications in particular, unnecessary weight is highly undesirable. Consequently, some engine manufacturers specify that the current detectors not be included in the ignition system. This results in more costly and time intensive ground based testing, often without the temperature, engine, pressure or other factor that caused the engine problem in the first place. Furthermore, the use of a transformer with core can be very restrictive as to where the detector can be positioned in the engine, as well as where in the ignition system the discharge current can be detected. In some applications, it is desirable to know whether failure of proper current discharge is due to a problem in the exciter, the cable leads or the igniter. Current transformers and similar devices with cores are very difficult to position near the igniter, for example.
The need exists, therefore, for a pulsed current detector that can be conveniently used as an add-on feature for an ignition system, if desired, and that can simply provide a go/no-go indication of spark discharges without a significant weight increase. Such a detector also should be able to be disposed conveniently at different locations in the system such as at different points along the cable leads or at the igniter, for example.